The present invention relates to a fuel assembly for a boiling water reactor (BWR). In particular, the invention is concerned with a fuel assembly suitable for being loaded into a reactor core wherein a water gap width on a side (control rod side) where a control rod is inserted and a water gap width on a side (opposite to the control rod side) where a control rod is not inserted, are almost equal to each other. This type of core is usually called a “C-lattice core”.
Recently, in a light water reactor (LWR), improvement in fuel economy and reduction of spent fuel (reduction in the number of fuel assemblies discharged from the reactor and reduction in the amount of spent fuel) have received considerable attention. To this end, it is effective to enhance discharge burn-up (high burnup) of fuel assemblies.
For the high burnup, it is necessary to increase uranium enrichment which means uranium-235 enrichment. The increase in uranium enrichment causes an increase in the hot-to-cold reactivity swing and a decrease in the reactor shut-down margin. The hot-to-cold reactivity swing is a reactivity difference of the reactor between a hot operating condition and a cold shut-down condition.
In addition, the increase in uranium enrichment causes an increase in the exposure (burnup) difference between fuel assemblies due to a residence (loaded) period difference in a reactor core. This increase in the exposure difference causes an increase in the maximum thermal power (or channel peaking factor) of fuel assemblies and a decrease in thermal margin. The thermal margin is a difference between an operating limit of thermal power and an actual maximum thermal power in an operating condition.
Japanese Laid-open No. Hei 8-285977 describes a short-length (part-length) fuel rod containing a burnable absorber, which absorbs thermal neutrons causing the nuclear reaction and is disposed at a corner position in the second layer from the outside of a fuel assembly to thereby enhance the thermal margin while attaining high burnup.
Japanese Laid-open No. Sho 64-31091 describes a fuel assembly for being loaded into a D-lattice core wherein a water gap width on a control rod side is larger than that on an opposite side, wherein an inner water gap is formed to increase the reactor shut-down margin, and the inner water gap or a water rod is shifted toward the side opposite to the control rod side to ensure the thermal margin.
However, in JP 8-285977, no consideration is given to improving of the reactor shut-down margin, and in JP 64-31091, a fuel assembly is disclosed that has a D-lattice core. Accordingly, no consideration is given to a fuel assembly for a reactor core wherein the water gap width on the control rod side and that on the opposite side are almost equal to each other.